Ganesh final report

“SUPERCRITICAL TECHNOLOGY IN POWER PLANT”

1 INTRODUCTION

1.1 Basic Rankine Cycle:

The Rankine cycle is the oldest functional heat cycle utilized by man. The

Rankine cycle is the very a basic vapor power cycle which is adopted in all the

thermal power plants. It is a four step process (Figure 1.1) which involves the

heating of the working fluid to its saturation temperature and vaporizing it

isothermally, expanding the vapor on a turbine (work cycle), condensing the

steam isothermally to the liquid phase and pumping it back to the boiler.

Figure 1.1.1 Basic Rankine Cycle

Figure 2 represents the temperature-entropy diagram for the simplest version of

the Rankine cycle. Although this simple version is rarely used it gives a very clear

and simple picture on the working of the cycle.

Process 1-2 is the pumping of the working fliud (water) into the boiler

drum. The power required is derived from the overall power developed. Process

2-3 is the heating of the water upto its saturation temperature (100°C at 1 atm

pressure for water) is reached and then isothermal heating of the water where the

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phase change from liquid to vapor occurs. Points 3 lie on the saturated vapor line.

The steam here is completely dry. Process 3-4 is the adiabatic expansion of the

vapor/steam on the turbine to obtain mechanical work. It is an isentropic process.

The temperature of the steam is reduced and it falls below the saturated vapor

line. The dryness fraction is reduced to less than one and a mixed liquid vapor

phase is present. Process 4-1 is the condensation process. This mixture is

condensed in a condenser isothermally and brought to the liquid phase back to the

pump.

FIGURE 1.1.2 Temperature vs. Entropy diagram for Rankine cycle

The steam is however, usually, superheated so as to obtain more work output.

Increasing the superheat to greater extent would lead to more work output.

However the energy spent in superheating the fuel is also high. The overall effect

is an increase in the thermal efficiency since the average temperature at which the

heat is added increases. Moisture content at the exit of the steam is decreased as

seen in the figure 1.3.

Superheating is usually limited to 620°C owing to metallurgical considerations.

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Figure1.1.3 Rankine cycle with superheating

1.2 Energy Analysis of the Rankine Cycle:

All four components in the Rankine Cycle (pump, boiler, turbine and

condenser) are steady flow devices and thus can be analyzed under steady flow

processes. K.E and P.E changes are small compared to work and heat transferred

and is thereby neglected.

Thus the steady flow equation (per unit mass) reduces to:

Q+hini = W+hfinal

Boiler and condenser do not involve any work and pump and turbine are assumed

to be isentropic. The conservation of Energy relation for each device is expressed

as follows:

Steam turbine:

As the expansion is adiabatic (Q=0) and isentropic (S3=S4), then,

W3-4=Wturbine= (h3-h4) kJ/kg

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Condenser:

Heat rejected in the condenser, Q4-1+h4=h1+W4-1

Since W4-1=0, Q4-1=h1-h4

Thus,

Q4-1=-(h4-h1) kJ/kg

Pump:

Work required to pump water:

Wpump=h1-h2 kJ/kg (-ve work)

Boiler:

Heat added in boiler:

Q2-3=h3-h2 kJ/kg=h3-h1-Wpump kJ/kg

Thus, the Rankine Efficiency=Work done/Heat added

= (h3-h4-Wp) / (h3-h1-Wp)

Neglecting feed pump work as it is very small compared to other quantities, the

efficiency reduces to:

ηrankine= (h3-h4) / (h3-h1).

1.3 Factors increasing the Rankine Efficiency:

i. Lowering the condenser pressure:

Lowering the condenser pressure would lead to the lowering of

temperature os steam. Thus for the same turbine inlet state, more work is obtained

at lower temperatures.

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This method though cannot be extensively used as it reduces the dryness

fraction x of the steam. This is highly undesirable as it decreases the turbine

efficiency is reduced due to excessive erosion of the turbine blades.

ii. Superheating the steam to high temperature:

There is an increase in the work output if superheating of steam is done. It

increases the thermal efficiency as the average temperature at which heat is added

increases.

There is also another benfit of superheating; the steam at the exit of the

turbine is drier than in case of non superheated steam.

iii. Increasing the boiler pressure:

Increasing the boiler pressure raises the average temperature at which heat

is added and thereby increases the theramal efficiency. However the dryness

fraction decreases for the same exit temperature of the boiler. This problem can be

solved by employing reheating procedure. If however the boiler pressure is raised

to supercritical point greater efficiency is obtained as the latent heat absorbed

during phase change is reduced to zero.

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2 SUPERCRITICAL RANKINE CYCYLE

2.1 Supercritical technology:

When temperature and pressure of live steam are increased beyond the

critical point of water, the properties of steam will change dramatically. The

critical point of water is at 374 °C and 221.2 bar (218 atm), Figure 2.1, and it is

defined to be the point where gaseous component cannot be liquefied by

increasing the pressure applied to it. Beyond this critical point water does not

experience a phase change to vapor, but it becomes a supercritical fluid.

Supercritical fluid is not a gas or liquid. It is best described to be an intermediate

between these two phases. It has similar solvent power as liquid, but its transport

properties are similar to gases.

Figure 2.1.1 Phase diagram of water

2.2Efficiency:

The Rankine cycle can be greatly improved by operating in the

supercritical region of the coolant. Most modern fossil fuel plants employ the

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supercritical Rankine Steam Cycle which pushes the thermal efficiency of the

plant (see equation 4) into the low to mid 40% range.

ηsupercritical = (h2-h1-h3+h4 )/( h2-h1) -(eqn 4)

2.2 Definition:

Figure 2.2.1 T-S diagram for supercritical Rankine cycle

For water, this cycle corresponds to pressures above 221.2 bar and

temperatures above 374.15°C (647.3 K). The T-S diagram for a supercritical cycle

can be seen in Figure 6. With the use of reheat and regeneration techniques, point

3 in Figure 2.1, which corresponds to the T-S vapor state of the coolant after it has

expanded through a turbine, can be pushed to the right such that the coolant

remains in the gas phase. This simplifies the system by eliminating the need for

steam separators, dryers, and turbines specially designed for low quality steam.

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Material Concerns:

The primary concern with this cycle, at least for water, is the material

limits of the primary and support equipment. The materials in a boiler can be

exposed to temperatures above their limit, within reason, so long as the rate of

heat transfer to the coolant is sufficient to “cool” the material below its given

limit. The same holds true for the turbine materials. With the advent of modern

materials, i.e. super alloys and ceramics, not only are the physical limits of the

materials being pushed to extremes, but the systems are functioning much closer

to their limits. The current super alloys and coatings are allowing turbine inlet

temperatures of up to 700°C (973 K). the fourth generation super alloys with

ruthenium mono-crystal structures promise turbine inlet temperatures up to

1097°C (1370 K). Special alloys like Iconel 740, Haynes 230, CCA617, etc. are

used.

The metallurgical challenges faced and solutions:

Normal Stainless steel proves of absolutely no use in building SC and USC

Boilers.

The high temperature and pressure in the boiler induce huge amount of stresses

and fatigue in the materials. Also chances of oxidation are very high at such high

temperature and pressure.

To resist these stress levels and oxidation different advanced materials and alloys

should be introduced.

Also they should me machinable and weldable. This is a great metallurgical

challenge.

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3 DESIGN AND WORKING

3.1 Boiler Design:

The design of Super and Ultra supercritical boilers (also called as Benson

Boiler) is very critical as the working pressures of these boilers are very high. The

boiler shells, the economizer unit, super heaters, air preheaters are specially

designed. Their location is also of great significance.

i. Boiler shell:

As shown in the figure 3.1 the geometry of the boilers and the

configuration of the inlets determine the recirculation pattern inside boiler. The

intensive recirculation created in the symmetric boiler results in a more uniform

temperature field, lower temperature peaks, moderate oxygen concentration and

complete burnout of the combustible gases and char

Fig 3.1.1 Predicted Recirculation inside the combustion chamber

Table 3.1 lists the peak temperatures and burnout for designs A, B and C. the table also

lists the standard deviations of the predicted temperature and oxygen fields. The lowest

values for C indicate the higher degree of homogeneity. Thus the symmetrical boiler

seems to be the most suitable design.

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ii. Location of burners:

The number of burners in the boiler shell is also of prime importance.

Amongst all of them the downfired boilers are most suitable and advantageous.

Table 3.2 gives a clear idea.

iii. Boiler dimensions:

One of the most important advantages of HTAC applications are high heat

fluxes. Thus, compact combustion chambers can be built and the investment costs

can be lowered. The fourth calculation series was carried out in order to find the

combustion chamber dimensions which can, on one hand, ensure an efficient heat

exchange between combustion gas and water/steam mixture and on the other

hand, ensure high values of firing density. Three different sizes are tested and

they are named in as the small boiler, the medium size boiler and the large

boiler .It has been observed (see Table 3.3) that the small boiler is too short. At

the top a region of high temperatures exists and its enthalpy cannot be efficiently

used. On the contrary, in the large boiler although the heat fluxes are uniform,

they are two times lower than in the medium size boiler. Therefore, the medium

size boiler configuration is chosen for further investigations.

Small boiler Medium size boiler Large boiler

Firing Density

kW/m3

774 238 89

Outlet temperature,

K

1805 1558 1299

Table3.1.1 Results of the boiler size determination

3.2 Working:

As already discussed, the working of Supercritical Boilers is similar to the

working of sub-critical boilers. It works on the supercritical rankine cycle. Most

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supercritical boilers are being run at operating pressures above of 235 bars. The

working of ultra supercritical boilers has operating pressures above 273 bars

4 MATERIAL SELECTION

4.1 Metallurgical Problems:

The available materials today like stainless steel which are usually used

for boiler parts are not suitable for SC and USC boilers. They do not have the

enough creep strength to resist the high pressure. Also there is high rate of

oxidation at such high temperature and pressures which are beyond the capability

of these materials to resist. Capable, qualified materials must be available to the

industry to enable development of steam generators for SC steam conditions.

Major components, such as infurnace tubing for the waterwalls, superheater/

reheater sections, headers, external piping, and other accessories require

advancements in materials technology to allow outlet steam temperature increases

to reach 760°C (1400F). Experiences with projects such as the pioneering Philo

and Eddystone supercritical plants and the problems with the stainless steel steam

piping and superheater fireside corrosion provided a valuable precautionary

lesson for SC development. Industry organizations thus recognized that a

thorough program was required to develop new and improved materials and

protection methods necessary for these high temperature steam conditions.

4.2 Materials used:

The materials used should be sustainable to the very high pressure being

developed and should not get oxidized due to the very high temperature. Different

high temperature materials are being used like 9 to 12% ferritic steels T91/P91,

T92/P92, T112/P122 steel, Advanced Austenitic alloys TP347, HFG, Super 304,

Nickel and chrome-nickel super alloys like Inconel 740.

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Table 4.2 gives a very brief idea about the boiler materials used for

different parts of the boiler.

Heat surface Tube material Header material

Economiser SA-210 C SA-106 C

Furnace Walls SA-213 T12 SA-106 C

Super

heater/Reheater

SA-213 T12

SA-213 T23

SA-213 TP 304H

SA-213

TP347HFG

SUPER 304H

SA-335 P12

SA-335 P91

SA-335 P911

Steam Piping SA 335 P91

Table 4.2.1 Materials for different boiler parts

The materials for the other parts of the power plant (like turbine) also must be

sustainable for the super critically heated steam. The following table gives a detail idea

on the turbine materials of a plant operating on a supercritical cycle. (Table 4.3)

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Table 4.2.2 Materials for other parts

The following figures show some of the materials used for SC and USC boilers. Iconel

740 is widely used for steam pipings in almost all of them.

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Component 1,050° F 1,150 °F 1,300° F 1,400 °F

Casings

(shells, valves, steam chests, nozzles)

CrMoV (cast)

10CrMoVMb

9–10% Cr (W)

12CrW (Co)

CrMoWVNbN

CF8C-Plus

CCA617

Inconel 625

Nimonic 263

CCA617

Inconel 740

CF8C-Plus

Bolting 422

9–12% CrMoV

Nimonic 80A

9–12% CrMoV

CrMoWVNbN

Nimonic 105

Nimonic 115

Waspaloy

Nimonic 105

Nimonic 115

U700

Rotors/Discs 1CrMoV

12CrMoVNbN

9–12 % CrWCo

12CrMoWVNbN

CCA617

Inconel 625

CCA617

Inconel 740

Nozzles/Blades

422

10CrMoVNbN

9–12% CrWCo

10CrMoVCbN

Wrought Ni-based

Wrought Ni-based


Figure 4.1 TP347HFG Figure 4.2 Iconel 740

5 SUPERCRITICAL BOILERS

5.1 A typical Supercritical Boilers:

Largest CFB and first supercritical CFB sold to date is the Lagisza 460 MWe

unit Ordered by Poludniowy Koncern Energetyczny SA (PKE) in Poland. The design is

Essentially complete with financial closing expected in the first quarter of 2006 at

which time Fabrication and construction will commence. The largest capacity units in

operation today are the two (2) 300 MWe JEA repowered units which were designed to

fire any Combination of petroleum coke and bituminous coals. The physically largest

Foster Wheeler boilers in operation are the 262 MWe Turow Units 4, 5, and 6 which

were designed to fire a high moisture brown coal. The design and configuration of

these units with Compact solids separators and INTREX™ heat exchangers were used

as the basis for the Lagisza design as well as for this study. The Lagisza design was

adjusted to accommodate a typical bituminous coal and the steam cycle.

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Figure 5.1.1 The Lagisza 300 MWe plant in Pola

5.2 Super and Sub Critical Boilers (comparative study):

There are many advantages of super critical boilers over normal

subcritical boilers, the prime advantage being the increased efficiency and reduced

emissions. There are many more advantages like no need of steam dryers, higher

operating pressures leading to more work output etc.

It is thus very important to have a comparative study of both the

boilers.

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Table 5.2.1 Comparison of sub and supercritical boilers

6 ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA

6.2 Supercritical Boilers in India:

There haven’t been any supercritical boilers in use in India so far. The European

countries, USA, Japan have been using supercritical technology since the last two

decades. However, there are upcoming projects to build power plants working under

the supercritical technology in India.

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Technology Efficiency (%) Steam

pressure/temperature

Typical emissions

Ultra Supercritical

33–35

>242 bar and 593.33°C SO2-0.408 kg/MHh

NOx-0.286 kg/MWh

CO2-0.96 T/MWh

Supercritical

36–40

>221.2 bar and 537°C SO2-0.431 kg/MHh

NOx-0.304 kg/MWh

CO2-1.02 T/MWh

Subritical

42–45

165 bar 537°C SO2-0.445 kg/MHh

NOx-0.31 kg/MWh

CO2-1.02 T/MWh


The National Thermal Power Corporation (NTPC) had entrusted a techno

economic study to M/s EPDC for super-critical Vs Sub-critical Boilers for their

proposed Sipat STPS (4x500 MW) in Madhya Pradesh.

M/s EPDC has recommended that a first step to the introduction of super-

critical technology, the most proven steam conditions may be chosen and the most

applicable steam conditions in India shall be 246 kg/cm2, 538° C/566° C. With these

steam parameters, M/s EPDC has estimated that the capital cost for a supercritical

power station (4x500 MW) shall be about 2% higher than that of sub-critical power

plant but at the same time the plant efficiency shall improve from 38.64% to 39.6%.

Being a pit head thermal power project, the saving in fuel charges is not justified by

increase in fixed charges.

Here are some upcoming projects in India:

North Karanpura, Jharkhand – 3x660 MW

Darlipali, Orissa – 4x800 MW

Lara, Chattisgarh – 5x800 MW

Marakanam, Tamilnadu – 4x800 MW

Tanda-II, Uttar Pradesh - 2x660 MW

Meja, Uttar Pradesh - 2x660 MW

Sholapur – 2x660 MW

New Nabinagar-3x660 MW

Many more projects including 800 MW ultra super critical units under

consideration

7 CONCLUSION

The supercritical Rankine cycle, in general, offers an additional 30% relative

improvement in the thermal efficiency as compared to the same system operating in the

subcritical region. The cycle has been successfully utilized in fossil fuel plants but the

current available materials prohibit reliable application of the supercritical cycle to

nuclear applications. There is much work to be done in order to advance materials to

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the point where they will be able to reliably withstand the stresses of a supercritical

environment inside a nuclear reactor for a designed life span of 60 years.

Supercritical boiler technology has matured, through advancements in design

and materials. Coal-fired supercritical units supplied around the world over the past

several years have been operating with high efficiency performance and high

availability.

REFERENCES

1. “Design Aspects of the Ultra-Supercritical CFB Boiler”; Stephen J.

Goidich, Song Wu, Zhen Fan; Foster Wheeler North America Corp.

2. “Novel conceptual design of a supercritical pulverized coal boiler utilizing

high temperature air combustion (HTAC) technology”; Natalia Schaffel-

Mancini, Marco Mancini, Andrzej Szlek, Roman Weber; Institute of

Energy Process Engineering and Fuel Technology, Clausthal University of

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Technology, Agricolastr. 4, 38678 Clausthal-Zellerfeld, Germany; 6

February 2010.

3. “Supercritical (Once Through) Boiler Technology”; J.W. Smith, Babcock

& Wilcox, Barberton, Ohio, U.S.A.; May 1998.

4. “Steam Generator for Advanced Ultra-Supercritical Power Plants 700 to

760°C”; P.S. Weitzel; ASME 2011 Power Conference, Denver, Colorado,

U.S.A; July 12-14, 2011.

5. “Supercritical boiler technology for future market conditions”; Joachim

Franke and Rudolf Kral; Siemens Power Generation; Parsons Conference;

2003.

6. “Steam Turbine Design Considerations for Supercritical Cycles”; Justin

Zachary, Paul Kochis, Ram Narula; Coal Gen 2007 Conference;1-3

August 2007.

7. “Technology status of thermal power plants in India and opportunities in

renovation and modernization”; TERI, D S Block, India Habitat Centre,

Lodi Road, New Delhi – 110003.

8. “Applied Thermodynamics”; Dr. H.N Sawant; January 1992; revised July

2004.

9. “http://en.wikipedia.org/wiki/Boiler#Supercritical_steam_generator”

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Education

Ganesh final report